Sinuously folded quarter wave stripline directional coupler



Jan. 5, 1965 g L. OH 3,164,790

SINUOUSLY FOLDED QUARTER WAVE STRIPLINE DIRECTIONAL COUPLER Filed Feb.12, 1963 INVENTOR. [ll/.5 1.. 0/7

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w United StatesPatent O 3,164,790 SINUOUSLY FOLDED QUARTER WAVE STRIP-LINE DIRECTIONAL COUPLER Luis L. h, Seattle, Wash, assignor to TheBoeing Company, Seattle, Wash, a corporation of Delaware Filed Feb. 12,1963, Ser. No. 258,049 5 Claims. (Cl. 333--) This invention relates toan improved directional coupler for high-frequency electrical energytransmission systems and has for its general object to provide aneflicient directional coupler which is many times shorter thanconventional couplers operable at the same wavelength of applied energy.The invention is herein illustratively described by reference to thepresently preferred embodiment thereof; however it will be recognizedthat certain modifications and changes therein with respect to detailsmay be made Without departing from the essential features involved.

In particular it is an object hereof to devise a directional couplerhaving an electrical length which is approximately a quarter wavelengthat the center frequency of the operating band, yet is of practicablephysical length even at frequencies well below;200 megacycles persecond. For example, the improved coupler may be made as short asone-fourth or less the length of conventional directional couplers ofthe coaxial line or strip line type.

A further object is to achieve the described results in a device whichis relatively simple to construct, which is relatively inexpensive andwhich avoids problems of critical spacing and dimensioning ofcomponents.

A further object is to achieve such a device having high eificiency interms of high directivity, low reflectivity and low loss factor.Moreover, the achievement of relatively broad bandwidth though operatingon the principle of a quarter wavelength device is a further objective.Still another object is to devise a directional coupler compatible withminiaturized electronic systems in the VHF and UHF range, so as to besuitable for use in satellites and other space vehicles.

Still another object is to achieve the foregoing results in a couplerwhich lends itself well to manufacture by etching and laminationprocesses currently used in the successful manufacture of precisioncomponents of small size.

Directional couplers have a wide variety of uses including measurementof incident and reflected power, isolation or decoupling of componentsand selective power attenuation in high-power systems. Furthermore, theymay be used in the injection of unidirectional signals into transmissionlines.

In accordance with this invention a folded-line directional coupler ofthe quarter wavelength type achieving the foregoing objectives comprisestwo serpentine conductors of equal pitch mounted in mutuallysuperimposed, coincident relationship between parallel ground planesurfaces. Each such conductor mounted in insulatively spacedrelationship from the other conductor and from the adjacent ground planesurface has a total length (when stretched out) of substantiallyone-quarter wavelength at the mid frequency of the operating band. Inputand output transmission line or waveguide connectors are connected toboth ends of each serpentine conductor at the edges of the cavity spacedefined between the ground plane surfaces. It is found in practice thatsuch a coupler capable of operating efiiciently between 35 and 90megacycles per second may be as short as twelve inches whereas aconventional coupler operable at that frequency range would be of theorder of 48 inches in length. Tests with devices embodying the inventionrevealed directional coupling variations less than one and one-half dbover an octave frequency range, with directivity in excess of minus Ice33 db. Furthermore, it was determined that the novel coupler can beconstructed on any scale, simply by scaling quency range. The widthdimension may be the same for couplers of different length, except wherelength is changed to a major extent.

These and other features, objects and advantages of the invention willbecome more fully evident from the following description thereof byreference to the accompanying drawings.

FIGURE 1 is an exploded simplified isometric view of principalcomponents comprising the illustrative embodiment of the folded-linedirectional coupler.

FIGURE 2 is an isometric view of the assembled coupler.

FIGURE 3 is a fragmentary edge view of the coupler seen at right anglesto its length.

As illustrated the coupler comprises two conductive plates 19 and 12 ofplanar form which, in assembled relationship, define a cavity space Ctherebetween. Two elongated conductors 14 and 16 folded into a planarsinuous configuration are mounted in mutually superimposed relationshiupwithin the cavity space C. The undulations of each conductor occur atthe same pitch distance and preferably have the same amplitude ortransverse dimension. Mounted in the cavity space, the two serpentineconductors are disposed in congruent co-phased relation ship, that iswith the crests of one superimposed upon corresponding crests of theother. Each conductor when stretched out has a lineal lengthapproximately one-quarter wavelength at the mid-frequency of theintended operating band of the coupler. Coaxial transmission linefittings or connectors 14a and 14b are connected respectively toopposite ends of the serpentine conductor 14, and similar fittings 16aand 1611 are connected to the respective ends of conductor 16. Eachcoaxial line fitting comprises an outer shell and a central conductorand is or may be of conventional form, the outer shell being grounded tothe plates 1% and 12 and the central conductor being connected to therods 14 and 16.

As a matter of convenience the coaxial line fittings 14a and 16:: leadoutwardly from the cavity space C at respectively opposite ends of thecoupler whereas the coaxial line fittings 14b and 16b lead outwardlyfrom the cavity space at respectively opposite sides of the coupler atpositions near the opposite ends thereof.

For purposes of maintaining the two serpentine conductors 14 and 16 ininsulatively spaced relationship from each other and from the conductiveground plane members 10 and 12 a low-loss styrafoam panel 18 isinterposed between the serpentine conductors whereas thin Teflon sheets20 and 22 are interposed between conductor 14 and plate 10 and betweenconductor 16 and plate 12, respectively. These insulative layers may beof any suitable material and may in fact comprise plastic or otherdielectric material bases upon certain of which the serpentineconductors are formed as a conductive film or deposit produced by anetching process or other suitable techmque.

In operation, high-frequency energy fed into the coaxial line connector1611, for example, flows through and out connector 16b, with a smallportion of this incident energy being sampled by the line structure 14and appear ing in output connector 14b. Under these conditionspractically no power appears in connector 14a. Likewise radio-frequencyenergy fed into connector 16b flows through connector 16a, with a smallportion of this energy appearing at connector 14a and practically noenergy appearing at connector 14b. In representative cases a coupler ofthe invention only seven inches long operated successfully over abandwidth of 65 to megacycles per second and one twelve inches longoperated over a bandwidth from 35 to 90 megacycles per second. In eachcase the coupler was of the order of three inches wide and of the orderof one inch or less thick. By employing laminatingand etching processessizecan be further reduced .and performance further improved. In eachcase the directional coupling efficiency was of .the order of 33' db andbetter, with coupling variations less than one and one-half db.

These and other aspects of the invention will beevident'to those skilledin the art based upon the foregoing disclosure of the presentlypreferredembodiment, which is intended to illustrate and not delimit the novelsubject matter.

I claim as my invention:

1. A broadband directional coupler comprising two transmission lineconductors each substantially onequarter wavelength longat themid-frequency of the band, said conductors being of'sinuously foldedform of substantially equal pitch distance, a pair of ground planeconductors defining a transmission line space therebetween, meansmounting said line conductors in mutually superimposed substantiallyco-phased relationship sandwiched between said ground plane conductors,said line conductors being insulatively spaced from each other and fromthe respectively adjacent ground plane conductors, and transmission linecoupling means having co-operable elements connected to the ends of therespective transmission line conductors and to the ground plane cn-.ductors for conducting of energy to and from said transmission linesbase.

2. The broadband coupler defined in claim 1, wherein the ground planesurfaces and the sinuously folded line conductors are of planar form.

3. The directional coupler defined in claim 1, including thin sheets ofdielectric materialinterposedbetween the respective line conductors andthe adjacent ground plane surfaces and a dielectric panel interposedbetween the line conductors.

4. A folded line quarter-wavelength directional coupler comprising meansincluding two substantially parallel conductive ground plane surfacesforming an elongated cavity space of a length several times itsthickness and of a width exceeding its thickness, two elongatedquarter-wavelength conductors formed sinuously and disposed in mutuallysuperimposed substantially parallel relationship between said surfaces,said conductors extending lengthwise in said cavity space and eachhaving undulations substantially co-phased with those of the other, saidconductors being insulatively spaced from each other and from saidsurfaces, and energy transmission connector means coupled to therespective ends of said conductors and extending outwardly from saidcavity space.

5. The directional coupler defined in claim 4, wherein thefirst-mentioned means comprises two conductive plates and the energytransmission means comprises coaxial line.

connectors having outer shells grounded to the plates and centerconductors connected to the respective ends of the quarter-wavelengthconductors.

References Cited by the Examiner UNITED STATES PATENTS 2,749,519 6/56Kostriza et a1. 33310 2,892,163 6/59 Todd 33384 3,012,210 12/61 Nigg33310 HERMAN KARL SAALBACH, Primary Examiner.

1. A BOARDBAND DIRECTIONAL COUPLER COMPRISING TWO TRANSMISSION LINECONDUCTORS EACH SUBSTANTIALLY ONEQUATER WAVELENGTH LONG AT THEMID-FREQUENCY OF THE BAND SAID CONDUCTORS BEING OF SINUOUSLY FOLDED FORMOF SUBSTANTIALLY EQUAL PITCH DISTANCE, A PAIR OF GROUND PLANE CONDUCTORSDEFINING A TRANSMISSION LINE SPACE THEREBETWEEN, MEANS MOUNTING SAIDLINE CONDUCTORS IN MUTUALLY SUPERIMPOSED SUBSTANTIALLY CO-PHASEDRELATIONSHIP SANDWICHED BETWEEN SAID GROUND PLANE CONDUCTORS, SAID LINECONDUCTORS BEING INSULATIVELY SPACED FROM EACH OTHER AND FROM THERESPECTIVELY ADJACENT GROUND PLANE CONDUCTORS, AND TRANSMISSION LINECOUPLING MEANS HAVING CO-OPERABLE ELEMENTS CONNECTED TO THE ENDS OF THERESPECTIVE TRANSMISSION LINE CONDUCTORS AND TO THE GROUND PLANECONDUCTORS FOR CONDUCTING OF ENERGY TO AND FROM SAID TRANSMISSION LINESBASE.